1. Field of the Invention
The present invention relates to the preparation of non-stick (non-agglomerating) particles based on a crosslinked organopolysiloxane composition, especially elastomer particulates produced by polycondensation crosslinking, which particulates may contain an active principle encapsulated by and/or dispersed within the elastomer.
2. Description of the Prior Art
There is an increasing demand in this art for silicone elastomer particles with a view to their use in two principal types of applications:
(a) incorporation of these particles as filler material into a polymer matrix, in particular into a crosslinkable organopolysiloxane composition to produce an elastomer, with a view especially to an enhancement of flexibility, and PA1 (b) conditioning (encapsulation or dispersion) of an active principle by these particles, with a view to the protection and ultimately controlled release thereof. PA1 (A) 100 parts by weight of an .alpha.,.omega.-(dihydroxy)-polydiorganosiloxane; PA1 (B) 0.1 to 20 parts by weight of a crosslinking agent selected from among the organotrialkoxysilanes, the alkyl silicates and the products of the partial hydrolysis thereof; PA1 (C) a catalytically effective amount of a tin catalyst; PA1 (D) an effective amount of a surfactant of the oil-in-water type; PA1 (E) 3 to 100 parts by weight, preferably 5 to 30 parts, of a pyrogenic or precipitated silica powder; PA1 (F) water; and, if appropriate; PA1 (G) an active principle; and PA1 (i) it permits easy emulsification, in water, of the organopolysiloxane composition, even if this latter is a commercial composition in the form of two packages (or components), with or without fillers already incorporated into one of the two packages; and PA1 (ii) during stage (b), the organic solvent evaporates and the aqueous emulsion is then broken, which facilitates formation of the particles. PA1 (1) if the active principle is soluble in the volatile organic solvent, it can be introduced in solution in such solvent; PA1 (2) if the active principle is soluble in water, it is desirable that the water of emulsion be saturated with the active principle (in this manner, the major part of the active principle is homogeneously dispersed within the particles); or PA1 (3) if the active principle (G) is insoluble in water and in the volatile organic solvent, it is introduced in the form of particles dispersed in the organopolysiloxane composition, preferably in the starting silicone oils (A), these microparticles being encapsulated during the stage (b).
Among the numerous patents describing processes for the preparation of silicone elastomer particles or microparticles crosslinked by polyaddition and/or polycondensation reactions, the following are particularly representative:
EP-A-252,510, which describes a process comprising mixing the various constituents of a liquid silicone elastomer, which may contain a siliceous filler, at a temperature ranging from -60.degree. C. to +5.degree. C. and then atomizing the mixture in air heated to a temperature of from +80.degree. C. to 200.degree. C., in order to produce particles of substantially spherical shape and of a particle size ranging from a few tens of millimicrometers to several hundreds of micrometers;
U.S. Pat. No. 4,248,751, describing a process for the preparation of a silicone latex of polyaddition silicone elastomer particles dispersed in an aqueous phase, which entails polymerizing, in aqueous emulsion, a mixture of vinylated silicone oils, hydrogen-containing silicone oils and a surfactant, and then crosslinking such oils by heating after addition of a platinum catalyst (it is possible to add a colloidal silica to the latex in order to improve the properties of the film-forming elastomer covering obtained after coating of the latex onto a support and evaporation of the water, but this patent neither describes nor suggests the preparation of silicone elastomer particles having a particle size greater than 50 .mu.m from the latex emulsion);
EP-A-217,257, describing a process for the preparation of silicone particles of a particle size of from 0.01 to 10 mm, including emulsion-polymerizing a diorganocyclopolysiloxane bearing vinyl and mercaptoalkyl groups in the presence of an alkylarylsulfonic acid, breaking the emulsion by adding magnesium sulfate thereto and by heating, introducing a vinyl monomer and effecting a graft polymerization in the presence of a free radical initiator;
U.S. Pat. No. 4,594,134 which describes the preparation of microparticles or spherical particles, of a particle size ranging from 2 to 300 .mu.m, of silicone polyaddition elastomer, produced by atomization of a platinum-catalyzed composition, including an inhibitor of the platinum, in a dryer at about 230.degree. C. The crosslinking can be effected without platinum and under UV irradiation, but with a photosensitizer. Prior to crosslinking, the composition can be dispersed in a liquid medium such as an organic solvent or water;
Japanese Kokai 87/257,939, describing the preparation of a silicone powder by drying and atomization of an aqueous emulsion of a polyaddition silicone composition;
EP-A-267,003, which describes the preparation of microspheres of silicone elastomer by dispersing, in water in the presence of a surfactant, an organopolysiloxane composition which is crosslinkable by addition reactions of the MICHAEL type, and crosslinking of the composition;
U.S. Pat. No. 4,370,160, describing a process similar to EP-A-267,003, except that the organopolysiloxane composition is crosslinked under UV irradiation in the presence of a photosensitizer.